The biology of the mammalian pigment epithelium and its relationship to the photoreceptor cells is being studied in vivo and in vitro. Cytochemical studies of pigment epithelial cells of rats with inherited retinal dystrophy (RCS rats) have demonstrated four lysosomal enzyme activities - acid phosphatase, aryl sulfatase, N-acetyl-glucosaminidase and esterase activities. No differences were observed with respect to these enzymes between normal and RCS pigment epithelial cells. In vitro studies have been carried out on explant cultures of pigment epithelium from rat, rabbit, human, calf and lamb. Pigment epithelial cells of all species showed profuse apical processes as early as 30 minutes after being placed in culture. Morphology of apical processes varied with each species. Rat pigment epithelium phagocytosed latex spheres after 30 minutes in culture, while calf cells did not phagocytose latex at early time intervals despite the presence of apical processes. The reason for this latent period in calf is being investigated. Preliminary studies on explant cultures of 2 day old rat pigment epithelium indicate that the surface morphology differs from that of adult cells. In addition, latex spheres are not observed on the apical processes or within these cells, suggesting that the attachment phase of phagocytosis may be different at this early stage of development. Preliminary observations on the effects of cytochalasin B indicate that this drug induces rapid loss of apical processes in culture of rat pigment epithelial cells. Further studies on the effects of this drug are in progress.